bpf: define INT8_TYPE as signed char

Change the BPF backend to define INT8_TYPE with an explicit sign, rather
than a plain char.  This is in line with other targets and removes the
risk of int8_t being affected by the signedness of the plain char type
of the host system.

The motivation for this change is that even if `char' is defined to be
signed in BPF targets, some BPF programs use the (mal)practice of
including internal libc headers, either directly or indirectly via
kernel headers, which in turn may trigger compilation errors regarding
redefinitions of types.

gcc/

	* config/bpf/bpf.h (INT8_TYPE): Change to signed char.

gcc/config/bpf/bpf.h

@@ -99,7 +99,7 @@
 
 #define SIG_ATOMIC_TYPE "char"
 
-#define INT8_TYPE "char"
+#define INT8_TYPE "signed char"
 #define INT16_TYPE "short int"
 #define INT32_TYPE "int"
 #define INT64_TYPE "long int"